Although zinc alkaline cells display desirable characteristics in terms of high anodic voltage and high energy content per unit weight and volume, the zinc or zinc oxide-containing anodes of such cells display the unusual and disadvantageous characteristic of "shape change" over repeated discharging and recharging. "Shape change" generally refers to a gradual change in the shape of the anode, particularly the thickness of the active layer. The change is manifested as a gradual reduction in the thickness of active material at or near the edge of the anode and a gradual thickness increase toward the center. The thickness change is believed to result from disproportionate dissolution of the zincate ions by the electrolyte from the active material adjacent the edges of the anode and disproportionate reformation of zinc toward the center. In the process the binder material (which is commonly mixed in with the active material) tends to accumulate at the surface of the electrode. As the "shape" of the active layer thus changes, the performance of the cell diminishes. Whatever the precise mechanism, shape change results in reduced performance, in terms of energy content (ampere-hours).
Various proposals have been made for minimizing or reducing the undesirable effects of shape change, see for example U.S. Pat. Nos. 3,838,367 and 3,876,470, but they have been relatively complex or expensive to carry out. By reason of that undesirable affect, there has been a need for a zinc anode construction in which the shape change effect is reduced without serious cost penalties. The present invention accomplishes this result.
In a conventional zinc anode the active material is disposed on a current collector, on one or both sides of the current collector. This structure is wrapped or enclosed within a separator, or in any event is separated from the cathode in a cell by a separator. The current collector is usually of copper in a foraminous form such as a woven screen, an expanded sheet, or a die-cut sheet. The active layer is predominantly zinc and/or zinc oxide, and usually will comprise a mixture of zinc oxide with a lesser proportion of metallic zinc particles. Binders and various additives are sometimes included. The separator in which the active material/current collector structure is wrapped or encased, in an insulative, electrolyte-permeable material, usually a sheet material, and typically is a thermoplastic or thermoset. As assembled to form a cell or battery, one or more such anodes is disposed adjacent or between cathodes, e.g., of nickel oxide or silver oxide. The electrolyte is typically potassium hydroxide. For further background relating to conventional zinc oxide anodes for secondary cells, reference may be had to Falk & Salkind, "Alkaline Storage Batteries."